The implementation handles multibyte characters correctly. It's likely that this could be optimized for the more common case where string and fill do not contain multibyte characters. Fixes ...

In PostGIS and Oracle, lpad('A', 5, 'x') produces 'xxxxA'. In the expression editor in QGIS, it produces 'Axxxx'. Similarly, rpad() adds extra characters to the left, while PostGIS and Oracle add ...